1. Field of the invention
This invention pertains to a solid imaging method and apparatus for fabricating an integral three-dimensional object from a multiplicity of cross sectional portions of the object. More particularly, the cross sectional portions correspond to solidified portions of contiguous layers of a photoformable composition. The method and apparatus use a dispenser in a coating station, which transfers part of the photoformable composition over the free surface of the composition so that a doctor blade may produce a uniform liquid layer.
2. Description of Related Art
Many systems for production of three-dimensional modeling by photoforming have been proposed. European Patent Application No. 250,121 filed by Scitex Corporation Ltd., on Jun. 6, 1987, discloses a three-dimensional modeling apparatus using a solidifiable liquid, and provides a good summary of documents pertinent to this art. U.S. Pat. No. 4,575,330, issued to C. W. Hull on Mar. 11, 1986, describes a system for generating three-dimensional objects by creating a cross-sectional pattern of the object to be formed at a selected surface of a fluid medium capable of altering its physical state in response to appropriate synergistic stimulation by impinging radiation, particle bombardment or chemical reaction. Successive adjacent laminae, representing corresponding successive adjacent cross-sections of the object, are automatically formed and integrated together to provide a step-wise laminar buildup of the desired object, whereby a three-dimensional object is formed and drawn from a substantially planar surface of the fluid medium during the forming process. U.S. Pat. No. 4,752,498, issued to E. V. Fudim on Jun. 21, 1988, describes an improved method of forming three-dimensional objects, which comprises irradiating an uncured photopolymer by transmitting an effective amount of photopolymer solidifying radiation through a radiation transmitting material which is in contact with the uncured liquid photopolymer. The transmitting material is a material which leaves the irradiated surface capable of further crosslinking so that when a subsequent layer is formed it will adhere thereto. Using this method, multilayer objects can be made.
A publication entitled "Automatic Method for fabricating a three-dimensional plastic model with photohardening polymer" by Hideo Kodama, Rev. Sci. Instrum. 52(11), 1770-1773, Nov. 1981, describes a method for automatic fabrication of a three-dimensional plastic model. The solid model is fabricated by exposing liquid photo-forming polymer to ultraviolet rays, and stacking the cross-sectional solidified layers. A publication entitled "Solid Object Generation" by Alan J. Herbert, Journal of Applied Photographic Engineering, 8(4), 185-188, August 1982, describes an apparatus which can produce a replica of a solid or three-dimensional object much as a photocopier is capable of performing the same task for a two-dimensional object. The apparatus is capable of generating, in photopolymer, simple three-dimensional objects from information stored in computer memory. A good review of the different methods is also given by a more recent publication entitled "A Review of 3D Solid Object Generation" by A. J. Herbert, Journal of Imaging Technology 15: 186-190 (1989).
Most of these approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e., exposing a photoformable composition with a laser beam according to a desired pattern and building a three-dimensional model, layer by layer. In addition to various exposure techniques, several methods of creating thin liquid layers are described which allow both coating a platform initially and coating successive layers previously exposed and solidified.
The aforementioned methods of coating, however, are not capable of ensuring flat uniform layer thickness or of producing such layers quickly, or they do not effectively prevent damage or distortion to previously formed layers during the successive coating process and they involve coating only liquid formulations of preferably low viscosity. Furthermore, they omit to recognize very important parameters involved in the coating process such as the effects of having both solid and liquid regions present during the formation of the thin liquid layers, the effects of fluid flow and rheological characteristics of the liquid, the tendency for thin photoformed layers to easily become distorted by fluid flow during coating, and the effects of weak forces such as hydrogen bonds and substantially stronger forces such as mechanical bonds and vacuum or pressure differential forces on those thin layers and on the part being formed.
The Hull patent, for example, describes a dipping process where a platform is dipped below the distance of one layer in a vat, then brought up to within one layer thickness of the surface of the photoformable liquid. Hull further suggests that low viscosity liquids are preferable but, for other practical reasons, the photoformable liquids are generally high viscosity liquids. Motion of the platform and parts, which have cantilevered or beam regions (unsupported in the Z direction by previous layer sections) within the liquid, creates deflections in the layers, contributing to a lack of tolerance in the finished part. In addition, this method is rather slow.
U.S. Pat. No. 2,775,758, issued to O. J. Munz on Dec. 25, 1956, and the Scitex application describe methods by which the photoformable liquid is introduced into a vat by means of a pump or similar apparatus such that the new liquid level surface forms in one layer thickness over the previously exposed layers. Such methods have the aforementioned disadvantages of the Hull method except that the deflection of the layers during coating is reduced.
The patent issued to Fudim describes the use of a transmitting material to fix the surface of a photopolymer liquid to a desired shape, assumably flat, through which photopolymers of desired thickness are solidified. The transmitting material is usually rigid and either coated or inherently nonadherent to the solidified photopolymer. The methods described by Fudim do not address the problems inherent in separating such a transmitting material from a photopolymer formed in intimate contact with the surface of the transmitting material. Whereas the effects of chemical bonding may be reduced significantly by suitable coatings or inherently suitable films, the mechanical bonds along with hydrogen bonds, vacuum forces, and the like are still present and in some cases substantial enough to cause damage or distortion to the photopolymer during removal from the transmitting material surface.
Methods utilizing doctor blades and/or material supply mechanisms have been proposed in such publications as Japanese Patent Application Publication numbers 61-114817, 61-114818, and 61-116322. However, these methods require an exact amount of material or photoformable composition to be added in the vessel every time a layer has to be formed. Also, they require the doctor blade or smoothening blade to have a length equal to the width of the vessel in order to properly operate. Because of this, the systems described in these patents have restrictions necessarily confining the photosensitive material between the doctor blade and part of the vessel at all times. Thus, it becomes very difficult to form a uniform layer in one continuous pass of the doctor blade without ending up with an excess or shortage of material at the end of the pass. In other words, the doctored layer may be either lacking a part of it at the end of one doctoring operation or it may have an excess of material, which will be very difficult to redistribute in order to achieve the proper thickness and uniformity, due to the confined nature of the arrangement. Also, the doctor blade has a tendency to create wave motion in the material surrounding the previously exposed layer causing a disturbing effect, particularly on parts of the previously exposed layer which are partially unsupported.
Thus, it is one of the objects of the present invention to provide an apparatus and a method for fabricating an integral three-dimensional object from a multiplicity of cross sectional portions of the object, the cross sectional portions corresponding to solidified portions of contiguous layers of a photoformable liquid composition, in a fast and uniform manner. Another object of the present invention is to provide a gentle way of raising part of the photoformable composition above the surface of said composition and in front of the doctor blade. Use of a pump to recirculate a liquid of the nature used in solid imaging or stereolithography does not present a viable solution because the viscosity and mainly sensitivity of such compositions cause blockage of the paths and seizure of the pumping operation at an unacceptably high rate. Premature polymerization within the higher-shear components of the pump seem to be the most probable cause of this problem.